The present invention refers to industrial and medical radioscopy (X-ray inspection) and especially to a fibre optic X-ray camera which, due to its structural design, is suitable for use under a high level of irradiation. This property is of importance for many industrial applications where the service life of conventional X-ray cameras is insufficient.
FIG. 2 shows a typical fibre optic X-ray camera composed of a scintillator device 20, a fibre optic system 22 and a semiconductor sensor 24. The semiconductor sensor 24 normally consists of CCD sensors or photodiode arrays. The scintillator device 20 converts an X-radiation 26, which impinges on the X-ray camera and which has passed e.g. an object 28 to be examined, into visible light whereupon the fibre optic system 22 conducts the light produced in the scintillator device 20 onto the sensor 24 which provides detection of the light with local resolution. The fibre optic system 22 replaces here a conventional lens optic system, since, in comparison with a conventional lens optic system, the fibre optic system 22 entails substantially lower losses of light; this has the effect that the sensitivity of the fibre optic X-ray camera is much higher than that of a camera comprising a lens optic system.
Fibre optic X-ray cameras can advantageously be used for all applications where a compact structural design or a high local resolution is required. As shown in FIG. 2, the fibre optic system 22 can be implemented as a so-called taper in the case of which an image of the input window is formed on a smaller output window.
However, in view of their comparatively short service life under a high level of irradiation, fibre optic X-ray cameras have not yet gained importance for industrial applications.
The reason for the short service life of this kind of camera under a high level of irradiation is to be seen in the fact that only part of the X-radiation 26 is absorbed by the scintillator device 20. In the insulator material of which the fibre optic system 22 consists, normally glass, colour centres are produced, whereby the fibre optic system 22 discolours and becomes brown and the quantity of light coming from the scintillator device 20 and arriving at the semiconductor sensor 24 decreases continuously. At a radiation dose of typically 100 kRad, the camera gets unfit for use. In the case of industrial applications, this dose may be reached within a few hours or days. According to the prior art, it was necessary to replace fibre optic systems whose quality had deteriorated in this way or to anneal them during a long interruption of operation.
U.S. Pat. No. 5,594,253 discloses a hybrid luminescent device for converting ionizing and penetrating energy, such as X-ray beams, gamma rays, neutrons, ions, electrons and the like, into visible light for display applications. The hybrid luminescent device comprises a phosphor screen arranged on the input surface of a fibre optic scintillator which can, in turn, be releasably coupled to a camera or to a some kind of recording medium.
JP 9-90039 A discloses a fibre optic radiation sensor making use of an optical fibre, which is formed by coating a core with a cover layer, and measuring the radiation dose in the vicinity of the fibre by detecting the light propagation loss of the fibre which occurs when defects are caused in the glass of the fibre by gamma rays which are present in the vicinity of the fibre. A hole extending in the longitudinal direction of the fibre is formed through the cover layer, and a metal wire, which produces heat when the wire is excited, is conducted through the hole so that the defects in the core and in the glass in the vicinity of the core can be healed effectively when the wire is excited.
U.S. Pat. No. 4,229,069 A discloses a device for remote viewing of objects located in areas with ionizing radiation fields. A fibre optic channel consisting of a bundle of fibres transmits the recorded image of an object to be viewed beyond a biological shield provided as protection against the ionizing radiation. A heat source implemented as a coil is provided for contributing to a thermal stabilization of the light conduction properties of the fibre bundle in the area with ionizing radiation by heating the fibre bundle.
It is the object of the present invention to provide a fibre optic X-ray camera which permits an improved detection of X-radiation for a long period of time.
It is a further object of the present invention to provide a method for an improved detection of X-radiation for a long period of time.
The present invention is a device for detecting X-radiation comprising:
a scintillator for converting X-radiation impinging thereon into light;
a detecting device for detecting the light produced by the scintillator;
a fibre optic system for conducting the light produced by the scintillator from an input end of the fibre optic system arranged at the scintillator to an output end of said fibre optic system, the detecting device being arranged in spaced relationship with the fibre optic system and an optical means being provided between the detecting device and the fibre optic system, said optical means directing the light emerging from the fibre optic system onto the detecting device; and
a heating means used for heating the whole fibre optic system during the detection of X-radiation to a predetermined temperature with a homogeneous temperature distribution.
The present invention is a further a method for detecting X-radiation comprising:
converting an X-radiation into light by means of a scintillator;
detecting by means of a detecting device the light produced by the scintillator; and
conducting the light from an input end of the fibre optic system arranged at the scintillator to an output end of said fibre optic system;
directing the light emerging from the fibre optic system onto the detecting device, which is arranged in spaced relationship with the fibre optic system, with the aid of an optical means provided between the detecting device and the fibre optic system; and
heating the fibre optic system with the aid of a heating means during the detection of X-radiation to a predetermined temperature with a homogeneous temperature distribution.
The present invention is based on the finding that the problems described, which arise when a fibre optic X-ray camera is used especially under a high level of irradiation, can be reduced by heating at least part of the fibre optic system to a predetermined temperature at which the colour-centre reduction rate vfxe2x88x92 is higher than or equal to the colour-centre production rate vf+, while the X-radiation is being detected. By means of this measure, the production of colour centres during the operation of the X-ray camera is avoided to a very large extent, whereby a discolouration of the fibre optic system is prevented.
Hence, one advantage of the present invention is to be seen in the fact that an exchange of the fibre optic system or an annealing of the fibre optic system, which would entail an interruption of operation, can be dispensed with completely or is at least only necessary after an operating period that is much longer than the operating periods which have hitherto been possible.
A further advantage which the invention described hereinbelow shows in comparison with the prior art is that it permits a substantially improved service life of the fibre optic X-ray camera under a higher levels of irradition. The longer service life allows the use of this type of X-ray camera under industrial conditions for the first time.
According to a further aspect of the present invention, an improved fibre optic X-ray camera and an improved method of detecting X-radiation are provided so as to permit a long-term operation of the fibre optic X-ray camera also under higher levels of irradiation, a mechanical stress on or a non-uniform radiation resistance of the fibre optic system caused by inhomogeneous temperature distribution in the fibre optic system being avoided. The detecting device is arranged in spaced relationship with the fibre optic system and an optical means is provided between the detecting device and the fibre optic system, the optical means directing the light emerging from the fibre optic system onto the detecting device.
During operation of a fibre optic X-ray camera, i.e. especially when the fibre optic system is heated, a sensor coupled directly to the fibre optic system is exposed to high temperatures due to the fact that the fibre optic system is heated strongly. This high temperature causes a high dark signal in the sensor, and this will result in a deterioration of the image quality and in reduced dynamics of the X-ray camera.
This problem can be solved by cooling the fibre optic system on the side facing the sensor. This solution is, however, disadvantageous insofar as, due to the resultant inhomogeneous temperature distribution in the fibre optic system, high mechanical stresses have to be expected, which may result in a formation of cracks and which may therefore cause damage to the fibre optic system.
This inhomogeneous temperature distribution in the fibre optic system is also disadvantageous insofar as, in the case of a disadvantageous diameter/length ratio of the fibre optic system, the strongly inhomogeneous temperature distribution in the fibre optic system will additionally cause a locus-dependent variation of the radiation resistance and, consequently, an irregular discolouration of the fibre optic system in certain areas thereof.
The present invention is based on the finding that the above-mentioned problems entailed by an inhomogeneous temperature distribution, especially under high levels of irradiation, can be eliminated by arranging the detecting device (the sensor) in spaced relationship with the fibre optic system, the light emerging from the fibre optic system being conducted via an optical means to the sensor. Since the fibre optic system can be heated uniformly throughout its whole length, a homogeneous temperature distribution will be achieved in the fibre optic system without influencing the electronic properties of the sensor, faster heating of the whole fibre optic system being additionally possible in this way. Furthermore, due to the homogeneous temperature distribution in the fibre optic system, the occurrence of mechanical stresses in the material of the fibre optic system will be avoided to a large extent, whereby mechanical loads on the fibre optic system, which might cause damage to the fibre optic system, will be reduced extremely.